Generally, a transmission amplifier of high efficiency used in a radio transmission device such as a movable body base station or the like has strong non linear property. Consequently, when a modulation signal for high speed radio communication is transmitted, non linear distortion in such a transmission amplifier causes out-of-band radiation power in a transmission modulation signal and the adjacent transmission channel is affected.
As a system for restraining out-of-band radiation by the transmission amplifier, a predistortion system for compensating non linear distortion in a transmission amplifier by adding a distortion signal having reverse property of the non linear distortion property of the transmission amplifier to an input signal and inputting to the transmission amplifier has been known. In particular, an adaptive pre diction system that adaptively executes distortion compensation by feeding back the output of the transmission amplifier at the input side can drastically restrain out-of-band radiation.
FIG. 10 is a principle diagram of the predistortion system. Normally, in the transmission amplifier, output is saturated and a linear signal with respect to an input signal is not output easily as the input power becomes larger (1001 of FIG. 10). The non linear property of the amplifier causes the adverse effect as described below.
FIG. 11 is an illustration diagram of deterioration of spectrum property caused by non linear property of the transmission amplifier.
As illustrated in FIG. 11, the non linear property of the transmission amplifier radiates a needles spectrum 1103 outside a signal band 1102 with respect to an amplifier input 1101. The out-of-band radiation power deteriorates the property of another system using an out-of-band frequency.
Further, a needles spectrum is radiated also in the signal band 1102 although hidden by the signal property in FIG. 11. This causes property deterioration of signal itself.
Further, since many of present digital modulation systems require linear amplifier property, it is not avoided to use a linear low input power part when using an amplifier having the aforementioned saturation property. This causes deterioration of power efficiency of the transmission amplifier.
Consequently, reverse property of the amplifier property is applied to the input signal of the transmission amplifier by suing a predistortion technique (1002 of FIG. 10). By adding non linear amplifier property thereto, in the transmission amplifier output, compensated linear property can be obtained as a result as illustrated in 1003 of FIG. 10.
A predistortion system using power series has been conventionally proposed as a system of the predistortion. This is a system in which a compensation operation in a predistortion unit 1201 at the pre stage of a transmission amplifier is executed by power series calculation to an input signal x as illustrated in FIG. 12.
That is, in FIG. 12, the predistortion unit 1201 executes a distortion compensation of a transmission amplifier 1205 by executing power series calculation to the input signal x.
The output of the predistortion unit 1201 is converted into an analog signal in a D/A converter 1201, and further is orthogonally modulated by a signal oscillated from a local oscillator 1204 depending on a transmission base station in an orthogonal modulator 1203.
The modulated transmission analog signal is power-amplified in the transmission amplifier 1205, and the output is supplied to a transmission antenna 1207 via a coupler 1206, and is transmitted therefrom. Further, the output of the transmission amplifier 1205 is fed back to the input side from the coupler 1206.
That is, the output of the coupler 1206 is down converted by a signal oscillated from a local oscillator 1209 depending on the transmission base station. Further, the output is returned to a digital signal by an A/D converter 1210 and thereafter returned to a base band by a demodulator not specifically illustrated.
As for a feed back signal Sfb(n) obtained by the result, an error signal e(n) between with a transmission signal Sref(n) delayed by a delay circuit not specifically illustrated is calculated in a subtracter 1211.
Then, by a coefficient update unit 1212, power series calculation coefficients a, b, c, d, and the like that are supplied to the predistortion unit 1201 are updated so that the error signal e(n) is minimized based on a calculation of minimum mean squared error (Least Means Square).
In this manner, the power series calculation coefficient is gradually converged to a predetermined value, and a power series calculation is executed to the input signal x in the predistortion unit 1201 by using the power series calculation coefficient converged to the predetermined value. Herewith, in the steady state, non linear property of the analog circuit unit is restrained with a high accuracy while keeping high power efficiency. Then, even when the non linear property is changed due to influence of temperature or frequency, the analog gain variation is detected by the feed back signal Sfb(n). Then, the value of the power series calculation coefficient is updated in the direction to compensate the variation in the coefficient update unit 1212, and the change of the property can be dynamically compensated.
Note that the above configuration has a configuration to a complex signal in reality.
In the aforementioned configuration of the conventional technique, for example, assuming that two sine wave signals (two tone signals) whose frequencies are separated by 2Δf are input to an amplifier model that is modeled by a power series.cos 2π(fc−Δf)t+cos 2π(fc+Δf)t
However, fc is carrier frequency. As a result, in the output signal expressed by a power series, only a signal component that is largely separated in frequency from the carrier frequency fc and that is suppressed by a filter of an analog unit or the transmission amplifier itself is included in the term of even-ordered power. On the contrary, in the third power term, a needless component which is fc±3Δf, and in the fifth power term, a needless component which is fc±5Δf is generated at the vicinity of the carrier frequency. Accordingly, the non linear distortion in the transmission amplifier 1205 can be modeled by a power series constituted by only an odd-ordered power term. Consequently, as illustrated in FIG. 12, it is general that the power series calculated by the predistortion unit 1201 is constituted by only an odd-ordered power term.
Hereafter, as a formula of a power series, a simple power series formula which is ax+bx3+cx5+dx7 will be described for simplification. In the distortion compensation in reality, it is general to use a more complicated type power in which Volterra power and a delay component are considered in order to modeling the property of the transmission amplifier 1205 more surly. The detail thereof is described in the following Non-Patent Document 1.                [Patent Document 1] Japanese Laid-Open Patent Application Publication No. 2001-268150        [Patent Document 2] Japanese Laid-Open Patent Application Publication No. 2002-335129        [Non-Patent Document 1] V. J. Mathews and G. L. Sicuranza: “Polynomial Signal Processing”, John Wiley & Sons, Inc. (2000)        [Non-Patent Document 2] S. Haykin: “Adaptive filter theory” Kagaku Gijutsu Syuppan (2001). (translated by Hiroshi Suzuki, et al)        [Non-Patent Document 3] V. Mathews: “Adaptive polynomial filters”, IEEE Signal Processing Magazine, pp. 10-26(1991).        